1. Field of the Invention
The present invention relates to a method for reorganizing a volume in an optical disk library unit or any other storage media library unit operating on an additional writing format.
2. Description of the Related Art
Saving data for a long time is used in a wide range of applications, such as drawings, slips, documents, forms, etc. To retain data for a long time, optical disks are often used due to their low bit cost (reduction in cost per unit storage capacity) and their long-term retention property. Particularly as the capacity of data increases, it is more often that an optical disk library unit is used to make up a large-capacity volume constituted by a plurality of disks.
An optical disk library unit is connected to a server of a personal computer or a work station through an interface such as an SCSI. Generally, a plurality of disks in the optical disk library unit are integrated and arranged into a large-capacity logical volume (hereinafter, referred to as “volume” simply). Thus, a user can access as one volume without recognizing the disks individually.
To integrate a plurality of disk surfaces into one volume in the optical disk library unit, any file is usually written like a postscript. That is, even when a file is updated by data overwriting or the like, the file is rewritten at the tail without using its original space. As a result, it is possible to write a file or to read an updated file without searching the plurality of disks. In the case where an optical disk library unit is used to make up a volume constituted by a plurality of disks, in addition to the low access performance of optical disks, there may occur an overhead due to a mechanical operation in the library unit. Therefore, it becomes an important factor on performance that searching the plurality of disks can be avoided in processing such as writing a file. Information as to which disk valid data belongs to and where the valid data is located in the disk is retained separately as file management information on a server. Thus, a user can acquire target valid data without accessing all the disks directly.
However, in an optical disk volume operating on an additional writing format, an invalid area increases in the volume as the quantity of updated or deleted files increases. In addition, the consumption of capacity in the optical disk volume operating on an additional writing format is faster than that of a volume using magnetic disks. Therefore, a system manager has to monitor the residual capacity of the volume periodically and endeavor to increase the capacity of the volume, for example, by adding a new medium to the volume, if the capacity is insufficient. Thus, the system manager is required to engage in such troublesome work. To avoid such work, it is necessary to relocate data in the volume automatically, for example, periodically.
In the related art, an automated method is adopted for relocating data in a volume constituted by a plurality of disks in an optical disk library unit. That is, invalid areas lying idle due to update or deletion are padded with valid data sequentially from a disk surface located at the head of the volume.
However, in this method, there is a problem that if the volume size increases due to increase in the number or density of disks, it takes enormous time to pad the disks with all the valid data. Further, access to the volume is inhibited during the processing for data relocation. Thus, there is a problem that it is inevitable to stop normal operation for about one or two weeks in accordance with the volume size.
In addition, in the method in which disk surfaces from the head of the volume sequentially are padded with valid data, written data is moved consequentially. Therefore, there is a fear that data is lost if the processing is interrupted carelessly due to a fault in hardware or the like during data relocation.
Further, in consideration of processing for reorganizing a volume, there occurs a problem in system resources occupation when a series of processes such as data relocation, deregistration of disk surfaces occupied only by invalid areas from the volume, formatting of the disk surfaces and addition of the disk surfaces to the volume are carried out. For example, at least two drives are occupied on read and write sides in data relocation, and one drive is occupied during formatting.